It is implied that all black cloaked monsters were once humans, but they became black cloaked monsters after their lives were taken from them by an unnatural and unpleasant cause, making them all un-dead.
They like to live in mansions that have no light and have been abandoned for years. Though it is unclear what happens if a person falls victim to a black cloaked monster, it may be speculated that the victim themselves will become un-dead and black cloaked.
If the speculation that black cloaked monsters are from people falling victim to already existing black cloaked monsters, it is unclear how black cloaked monsters came to exist to start with.
If all black cloaked monsters were once humans, it is unexplained how the first black cloaked monster ever came to exist. Seeing as logically the first black cloaked monster should have needed another black cloaked monster to turn themselves into one.